The curing rate of radiation curable compositions for coating applications or adhesive applications is a very important factor in determining the effectiveness of the composition. The problem with free radical curing systems is their inhibition in the presence of oxygen and thus these systems require costly inert gas facilities to ensure a satisfactory cure rate. On the other hand, cationic curing systems have no oxygen inhibition problems but have the problem of moisture sensitivity, i.e. the cure rate decreases with an increase in the relative humidity of air.
Cationic ultraviolet initiated photopolymerization of vinyl ethers in the presence of an onium salt initiator is well known in the literature. Desorcie et al. in U.S. Pat. No. 5,010,118 disclose the use of organic phenols in combination with polyaryliodonium salts, an epoxysilicone and an alkanediol to from a curable mixture. Specifically Desorcie et al. disclose the use of epoxysilicones, a polyaryliodoniumhexafluorometal or metalloid salt substituted with at least one nuclear bound alkoxy radical having at least 8 carbon atoms selected from the group consisting of diaryliodonium and triarylsulfonium salts, a compatibilizer which is a mixture of an 8-20 carbon alkyl phenol and a 4-12 carbon alkanediol to form a curable mixture when exposed to UV radiation.
Crivello in U.S. Pat. No. 4,108,747 discloses that aryl onium salts in combination with cationically polymerizable groups such as epoxides, vinyl ethers, or N-vinyl compounds are polymerizable under ultraviolet radiation. Specifically, the Crivello discloses the use of certain triphenyl sulfonium or diphenyl iodonium salts having anions derived from strong organic acids as photoinitiators for a variety of cationically polymerizable materials. The cationic polymerizable organic material can be selected from the group consisting of epoxy resins, vinyl organic monomers, vinyl organic prepolymers, cyclic organic ethers, cyclic organic esters, cyclic organic sulfides, cyclic amines and organosilicon cyclics in combination with an effective amount of triaryl onium salt of the general formula ((R.sub.a)(R.sup.1).sub.b (R.sup.2).sub.c S)+(XSO.sub.3)-- or ((R.sup.3).sub.d (R.sup.4).sub.3 I)+(XSO.sub.3)--.
Nelson, Carter, and Scranton, in the Journal of Polymer. Science.: Part A: Polymer Chemistry, Vol. 33, 247-256 (1995), disclose the photosensitization of the iodonium salt, bis(4-dodecylphenyl) iodonium hexafluoroantimonate, with anthracene in the photopolymerization of the divinyl ether, 3,6,9,12-tetraoxatetradeca-1,13-diene, commonly referred to as DVE-3.
Ciba-Giga Corporation in photoinitiator for UV Curing, A Formulator's Guide (p. 13), recommends the use of sensitizers when using their cationic photoinitiator Igracure.RTM. 261 (.eta..sup.5 -2,4-cyclopentadien-1-yl)[(1,2,3,4,5,6-.eta.)-(1-methyl ethyl)benzene]-iron(+)-hexafluorophosphate(-1).
Eckberg in EP0522703 A2 teaches the use of using reactive phenols such as 4-dodecylphenol alone or in combination with an epoxy functional organic compound preferably, 1,2 epoxydodecane and further containing a photosensitizer which is a benzophenone or diaryl ketone derivative as a diluent for iodonium photocatalysts. These compositions are useful for photopolymerizing epoxy functional silicones which are useful as release coatings.
In contrast to the above described compositions, the radiation curable compositions of this invention have a good curing rate in moist air without requiring the protection afforded by an inert gas facility.